Bitumen is a heavy type of crude oil that is often found in naturally occurring geological materials such as tar sands, black shales, coal formations, and weathered hydrocarbon formations contained in sandstones and carbonates. Bitumen may be described as flammable brown or black mixtures or tar-like hydrocarbons derived naturally or by distillation from petroleum. Bitumen can be in the form of a viscous oil to a brittle solid, including asphalt, tars, and natural mineral waxes. Substances containing bitumen may be referred to as bituminous, e.g., bituminous coal, bituminous tar, or bituminous pitch. At room temperature, the flowability of bitumen is much like cold molasses. Bitumen may be processed to yield oil and other commercially useful products, primarily by cracking the bitumen into lighter hydrocarbon material.
As noted above, tar sands represent one of the well known sources of bitumen. Tar sands typically include bitumen, water, and mineral solids. The mineral solids can include coal and inorganic solids such as sand and clay. Tar sand deposits can be found in many parts of the world, including North America. One of the largest North American tar sands deposits is in the Athabasca region of Alberta, Canada. In the Athabasca region, the tar sands formation can be found at the surface, although it may be buried two thousand feet below the surface overburden or more.
Tar sands deposits can be measured in barrels equivalent of oil. It is estimated that the Athabasca tar sands deposit contains the equivalent of about 1.7 to 2.3 trillion barrels of oil. Global tar sands deposits have been estimated to contain up to 4 trillion barrels of oil. By way of comparison, the proven worldwide oil reserves are estimated to be about 1.3 trillion barrels.
The bitumen content of some tar sands may vary from approximately 3 wt % to 21 wt %, with a typical content of approximately 12 wt %. Accordingly, an initial step in deriving oil and other commercially useful products from bitumen may typically require extracting the bitumen content from the naturally occurring geological material. In the case of tar sands, this may include separating the bitumen from the mineral solids and other components of tar sands.
One conventional process for separating bitumen from mineral solids and other components of tar sands includes mixing the tar sands with hot water and, optionally, a process aid such as caustic soda (see, e.g., U.S. Pat. No. 1,791,797). Agitation of this mixture releases bitumen from the tar sands and allows air bubbles to carry released bitumen droplets to the top of the mixture where a bitumen froth is formed. The froth may include around 60% bitumen, 30% water, and 10% inorganic minerals. The bitumen-enriched froth is separated from the mixture, sometimes with the aid of a solvent, and further processed to isolate the bitumen product. For example, the froth may be treated with an aliphatic (pentane-type) or an aromatic (naphtha-type) solvent to produce a clean bitumen product that may serve as a refinery upgrader feed stock. The bulk of the mineral solids can also be removed to form a tailings stream. The tailings stream may also include water, solvent, precipitated asphaltenes (in the case where the asphaltene is not soluble in the solvent used to separate the bitumen-enriched froth from the mixture), and some residual bitumen.
One significant disadvantage of the hot water extraction process is the quality and composition of the tailings produced by such a method. The tailings may include precipitated asphaltenes and/or residual bitumen, which represent unrecovered hydrocarbon material, and consequently, diminished yield. Additionally, the tailings produced by hot water extraction methods may include solvents and other materials that pose environmental hazards when disposing of the tailings. Furthermore, tailings produced by hot water extraction methods may have a sludge-like consistency requiring disposition in costly and potentially environmentally hazardous tailings ponds or other mechanisms.
Co-pending and commonly owned prior art U.S. application Ser. No. 12/041,554 discloses a method that addresses many of the problems identified above with respect to hot water extraction methods. The method utilizes a series of carefully selected hydrocarbon solvents to extract bitumen from bituminous material while avoiding such issues as asphaltene precipitation and the creation of sludge-like tailings. In the method, a first hydrocarbon solvent capable of complete or near complete dissolution of bitumen is mixed with the material comprising bitumen to create a bitumen-enriched solvent phase within the mixture of material comprising bitumen and first hydrocarbon solvent. The bitumen-enriched solvent phase is then displaced out of the mixture by adding further first hydrocarbon solvent to the mixture. While this step removes most if not all of the bitumen-enriched solvent phase from the mixture, in some embodiments, some of the first hydrocarbon solvent added to the mixture may remain entrained in the first mixture. In order to remove the entrained first hydrocarbon solvent from the mixture, a second hydrocarbon solvent that has a lower viscosity and is more volatile than the first hydrocarbon solvent is added to the mixture to displace the first solvent out of the first mixture. Any second solvent remaining in the mixture may be removed by heating the mixture to a temperature above the boiling point temperature of the second solvent. Relatively minimal energy is required to carry out this heating step due to the high volatility of the second solvent and the relatively low heat capacity of the inorganic phase present in the first mixture. The result of this method is a high yield of extracted bitumen and a tailings phase that has relatively little or no solvent content and a desirable water content.
One possible shortcoming of the above described method is that in the process of removing the first solvent from the mixture through the addition of the second solvent, a portion of the first solvent may leave the mixture as a mixture of first solvent and second solvent. In order to recover and reuse the first and second solvents in the method and thereby make the method more efficient, an additional separation step is required to separate at least a portion of the first solvent from the second solvent. Often, the separation step requires a distillation tower that is capable of separating the first hydrocarbon solvent from the second hydrocarbon solvent. Such distillation towers can be expensive to construct, maintain, and operate, and add a degree of complexity to the overall method.
Additionally, the ability of the second solvents disclosed in U.S. application Ser. No. 12/041,554 to at least partially dissolve bitumen may result in the second solvents being less effective as materials for displacing first solvent from the mixture. The second solvents may act more like dissolution agents than displacement agents, resulting in less than complete removal of the first solvent from the mixture.
The second solvents disclosed in U.S. application Ser. No. 12/041,554 may also be environmentally unfavorable. For example, the use of aliphatic hydrocarbons may result in the undesirable generation of greenhouse gases. Additionally, the aliphatic hydrocarbons may be less biodegradable and more expensive than other solvents suitable for use in bitumen extraction.
Further disadvantages in the above-described method may arise when liquefied petroleum gasoline (LPG) is used as the second solvent. Applicants believe that the gas phase of the LPG typically requires high capital costs and complex configurations that would not be necessary when using a liquid solvent. For example, the use of LPG may necessitate a pressure vessel that is complicated and expensive to build and operate. Additionally, the conditions required to flash LPG from the tailings typically result in the freezing of the water content in the tar sands. The ice formed may then subsequently interfere with the separation of the LPG from the tailings.